A pixel driving circuit of a traditional AMOLED is applicable to all types of transistors including a depletion type TFT. However, this pixel driving circuit does not have a threshold voltage compensation function, and thus cannot solve the problem of non-uniformity of the threshold voltage and non-uniformity of driving light emitting by an organic light emitting diode (OLED) caused by technique non-uniformity.
An oxide TFT is a development direction of a large-size AMOLED, because the oxide TFT has characteristics of depletion type, that is, the threshold voltage of N type is a negative value. The depletion type TFT adopts an AMOLED pixel driving circuit design of a traditional N type TFT. When compensating the threshold voltage by a diode connecting mode, since the threshold voltage is a negative value, TFT is turned off in advance for the source-drain voltage is zero before entering into sub-threshold saturation turn-off thereby losing the function of the threshold voltage compensation.